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Why do new jet engines cost billions to design?
This question might come off as crude because it involves people's salaries, but I do not understand why new designs of large turbofans cost many billions of dollars to design. For example, the Pratt & Whitney geared turbofan apparently cost 10 billion USD.
Why? Last I checked, most scientists and engineers do not make millionaire salaries. I think it's more around 100 to 250k tops. Even if you had 100 of them working for 10 years on it, that would be 250M, or a quarter of a billion dollars. Instead, jet engine designs seem to cost multi-billions.
I don't understand. If it's just designing a jet engine, then raw materials cannot possibly be a major factor even if it's something like titanium or composites. How many prototypes could you possibly need? I mean I hope it's not all trial and error.
The other thing is computer software, which I thought would make things easier and cheaper to design. Granted you cannot just input the desired thrust and press a button, but surely there is decent fluid dynamic software out there that can help you design something much easier and faster than before.
So what makes it so expensive? Is there some super costly certification process? Or do you somehow need more than 100 engineers collaborating? 1,000? 10,000? If so, why? Cannot imagine that.
Maybe someone can explain the general process of jet engine design in the first place because I'm sure that would be helpful. They way I imagine it, you just go through stage by stage and try to get each blade shape and diameter right.
Or maybe I have some misunderstanding and the 10 billion includes the costs to put up the factory?

You might want to read a bit about what goes into designing the materials & structure of a turbine blade: https://en.wikipedia.org/wiki/Turbine_blade

"computer software ..." - extremely correct programs for very small market are not exactly cheap... Even standard non-customized AutoCad will run more than 1K/person/year (even with all enterprise discount I doubt you get anywhere close to 3.99 Apple AppStore prices :) ) .

Comments have been cleaned up. Any attempt to answer the question in the comments will be deleted on sight

Even if we ignore the salaries of everyone involved; engineers, sales people, management, Q/A teams, manufacturing teams, more sales people, and then a few spare engineers...

I don't understand. If it's just designing a jet engine, then raw
  materials cannot possibly be a major factor even if it's something
  like titanium or composites.

While the raw materials alone are not necessarily costly, the materials, as well as the machining and processing, can be quite costly. This can be further compounded if the engine uses some kind of new material for which there is no effective production method yet. This was the case for the entire production of the SR-71 Blackbird which needed to figure out how to work with titanium before actually building anything. Even once the bespoke parts are produced for a new engine the company will then need to figure out how to produce the parts in sufficient quantity to produce the engines for market.

How many prototypes could you possibly need? I mean I hope it's not
  all trial and error.

It's not, but the FAA may require various demonstrations where they destroy the engine and you can bet the manufacturer is going to try that out before running any official tests. Once passed initial tests flying prototypes will need to be built and tested on real airframes that cost money, running jet fuel that costs money.

The other thing is computer software, which I thought would make
  things easier and cheaper to design. Granted you cannot just input the
  desired thrust and press a button, but surely there is decent fluid
  dynamic software out there that can help you design something much
  easier and faster than before.

It makes some things faster, like FEM, and surely makes complex routing easier, but like a good wrench, CAD software is a tool that makes things faster and easier. It does not do the work for you.

Numbers for fuel cost to run a modern engine at various power levels for a day of testing may be useful to reinforce this answer. - Also further consideration on costs like real estate and potential 'legacy costs' like paying out for shareholders or maintaining pension funds could be useful to highlight. Jet engine projects aren't typically done out of a lone engineer's garage after all... As they say, costs add up, and then you eventually have to pay people to add up those costs...

Never underestimate the costs to build the tools required to build a product.  Non-aviation example: the tooling required to manufacture an integrated circuit chip can cost more than a million dollars, and that's assuming you already own all of the manufacturing equipment.  Any time your tests find a flaw that requires a big change, you get to pay those costs all over again.

I see your 'raw materials' and I raise you titanium smithing.

Here is a cool link where it shows how engine are tested... they're tested on a plane built for testing engines...

Worth noting that because modern CAD makes things faster and easier, engineers are able to design more complicated things. You often don't end up spending less time designing things, you design things that just weren't feasible before.

@Nelson often they're tested on a 747. Since they are 4-engined it's safe enough to use one strut for testing. Examples: GE (video), Rolls Royce (which has both test locations).

An aspect you shouldn't forget is that in the aerospace sector a lot of tasks which in other sectors are done by less skilled/educated/paid workers are done by engineers for safety reasons. Aerospace is simple expensive because it is aerospace.

Another major cost is the facilities to test the engine including potentially an airstrip to fly from which will require constant maintenance.

@SethR That's a great point. In the maritime world, the first nuclear-powered submarine was designed and built in the 1950s in just about 4 years. Modern subs can be in development for over a decade!

@bta Exactly - Intel's woes with 10nm, and standard lithography vs DUV vs EUV come to mind.

Jet engines are some of the most complex machines ever created. They have to be as light, efficient, safe, and reliable as possible. There's a reason that most new airliners recently have been affected by delays from engine manufacturers. This is a hard balance to engineer when on a schedule and budget.
Jet engines could certainly be cheaper to develop and buy. You can get them at relatively "affordable" prices for remote controlled aircraft. But cost certainly increases with scale, and an aircraft owner expects an engine to run for thousands of hours with minimal maintenance while burning as little fuel as possible and not hurting anyone. Each new generation of engines has been more efficient than the last, and those improvements do not come for free.

If it's just designing a jet engine, then raw materials cannot
  possibly be a major factor even if it's something like titanium or
  composites.

It's not just the raw materials, but the processing involved. Modern engines push materials to their limits and beyond. Advanced manufacturing technologies have to be developed.
Let's say you have a new material or process you want to use. It can easily take at least hundreds of thousands of dollars just to develop one, and a new engine could include many of these. Even for a cheap raw material, the amount of labor required to create test articles, set up tests, run them, and document the results grows very quickly. You want to be sure you understand how the new material or process will work before moving forward with it. If things go wrong, you create big problems for your customers (aircraft manufacturers, and their customers).

How many prototypes could you possibly need? I mean I hope it's not
  all trial and error.

"Trial and error" is sometimes also called "science" which is what you need to develop new technologies. Obviously as testing progresses and the risks increase you'd like the "error" part to keep decreasing. But the trial part is very important for understanding how things will actually work (or not). This means not just full scale prototypes (which will go through several design iterations, even through airplane certification) but also subsystems and components. And you need to do enough tests to have statistical confidence that the results can be reliably reproduced.

The other thing is computer software, which I thought would make
  things easier and cheaper to design.

This is certainly true and these technologies have decreased the amount of physical testing that has to be done. But either way it's going to cost you money.
With products like jet engines, better tools does not generally mean "how cheap can we make this process" but "how much more performance can we get for the same money."

So what makes it so expensive? Is there some super costly
  certification process?

Yes. People like to fly on planes with engines that keep working and don't explode. This means rigorous regulations and certification. For the FAA, 14 CFR Part 33 covers the certification requirements for jet engines, to try to make failure events as rare as possible. Here are just some of the tests required by regulations:

Vibration
Overtorque
Calibration
Endurance
Overtemperature
Full operating range
System and component tests
Rotor lock
Full teardown
Blade containment/rotor unbalance
Rain, hail, and bird ingestion

Some of these tests are going to be destructive, either by design or by accident. Some of them are going to take a lot of time and effort. Just the paperwork involved with understanding all these requirements and documenting to the regulators that you've met them could easily take a good chunk of your 100 people.

Maybe someone can explain the general process of jet engine design in
  the first place because I'm sure that would be helpful. They way I
  imagine it, you just go through stage by stage and try to get each
  blade shape and diameter right.

It sounds like you have the basic idea. But engineering is about the devil in the details.
First, modern engines could have 20 or more stages, attached to 2 or 3 separate spools. The engineers have to decide the optimum number of stages and spools for the engine design. This means analyzing many different configurations, complexity tends to increase exponentially, as each stage affects the rest of the system.
Yes, the process is relatively simple if you're given static conditions to analyze. Of course it's important to optimize fuel consumption at cruise. But the engine still has to operate across a huge range of conditions. Then there are the dynamic conditions of acceleration and deceleration. The engine has to start and be stable in both crosswinds and tailwinds. It has to be able to start on the ground or in the air after getting extremely cold. Weird things can happen as things expand and contract with temperature.
If you're looking at simple analysis of how pressure and temperature change through a jet engine, there's probably a lot of hand waving about a stage called the "combustor" where you magically get an increase in temperature. The process of burning the fuel in the extreme conditions of a jet engine is extremely complex. The air rushing in the front has to be compressed, then slow down enough to not extinguish the flame. The flame has to be contained in the combustor section throughout operation, and not overheat the turbine stages behind it.
Higher temperatures and pressures provide better efficiency but materials are pushed to their limits. New superalloys and manufacturing techniques have to be perfected to create materials able to withstand extreme temperatures while spinning at thousands of RPM. They have to put small holes and passages into the blades to force out cooling air that covers the surface of the blade so it doesn't directly contact the extremely hot air in the turbine.
Then you also have mechanical energy being extracted by a generator, and pneumatic energy being extracted for the aircraft's bleed air system. The engine has to be able to cope with varying demands of these systems.
There's also the problem of various spools rotating and thousands of RPM and not causing too much friction heat or prematurely wearing out. Engineers need to understand the temperatures, aerodynamics, and rotational stress on each part, through the whole operating range of the engine, and how it affects the rest of the engine.
And it's not just enough to get something that works. Someone will always be asking the question, "How can we make this more efficient?" Modern engines are pulling many different tricks to squeeze out every bit of efficiency that they can. Air is bled off and vanes can be adjusted to make the engine stable in all operating conditions. New concepts and technologies are developed. Modern turbofans have the problem of a low pressure turbine in the back that needs to spin as fast as possible to be efficient connected to a fan in the front that needs to spin much slower to be efficient. For the Pratt & Whitney example you give, their solution was a gearbox to allow the two to turn at different speeds. This was a very difficult challenge that took them decades to finally get into an end product.
All of this complexity has to be managed by software that monitors an array of sensors throughout the engine and continually adjusts the many parameters to maintain stable and efficient operation. This software has to run on computers that will operate across a huge range of temperatures and under constant vibration.
You also have to keep in mind how all of these thousands of parts will be manufactured and then assembled, and then maintained through the life of the engine. You need people planning to ensure that a mechanic will have access to the right components with the tools they need, and what processes have to be followed to assemble and disassemble the various parts.
Then there are also collateral effects like noise and pollution. There will be engineers tasked with understanding how these are generated and how they can be reduced to acceptable levels with as little cost as possible.
This is just an overview of the many areas involved in designing a jet engine. There are certainly more, and each detail here could easily require a specialized team working on it.

Naturally, I just calculated that we need about 1,000 of those 50 lbs thrust RC engines (5,000$ each) to make an A320 fly. :-)

@PerlDuck Have you included in that calculation the higher fuel consumption of the engines, the additional weight of the engines and the additional fuel, and the higher fuel consumption due to that additional weight, and the additional weight of that fuel and the higher fuel consumption due to that additional weight...?

It's not just "modern" jet production that is sometimes delayed by engine design delays.  The Lockheed L-1011 was famously delayed by issues with Rolls Royce designing a specific engine for that airframe, and it was very painful financially for Lockheed (and Rolls Royce).

@Milwrdfan Even farther back, the JT9Ds required some redesign before 747 deliveries could start. They were apparently prone to fail at high power settings.

This is a great overview of the complexities of a modern 'jet' engine but I didn't see anything that showed how it all added up to US $10B. Estimating the number of people required for the specifications, design and redesign, test, facilities, etc. would help this answer (for me.)

@Alexander Of course I have not. It was merely a joke. I was just curious how many of those little ones we would need. Made me think of a fly with its thousands of tiny eyes compared to other animals with just two larger ones.

"People like to fly on planes with engines that keep working and don't explode." - Citation needed.

@CramerTV, number of people involved?  Lots.  For example, if you're using a new superalloy, you need to quantify the properties of the material: a mechanical-test technician to measure the strength, hardness, toughness, and so on, a chemist to confirm the composition, a machinist to convert bulk metal into test specimens, and a laboratory supervisor to coordinate things.  That's four people for just one aspect of one part of designing a jet engine.

Last I checked, most scientists and engineers do not make millionaire salaries. I think it's more around 100 to 250k tops. Even if you had 100 of them working for 10 years on it, that would be 250M, or a quarter of a billion dollars.

The salaries in aerospace average less than 100k, it ain't IT, but they're not the issue.
You can develop a basic experimental jet engine with 100 engineers and scientists. The thing is, you can't design and build a mass-produced turbofan engine with such a team.
You can with 1,000. But airlines and authorities want it reliable, and you can't build a reliable high-bypass turbofan with only 1,000. That takes thousands because of how thoroughly everything has to be validated and double-checked.
Building a competitive airliner engine is even harder. It would be about realistic today with 10,000 employees, but still a feat. The job isn't just engine parts design, most of the hard work is in trying out hundreds of materials in R&D, machine design, technology development, QM and QC development. All the things that contribute to producing good engines, and then producing them efficiently.
Saturn, one of the smaller jet engine builders today, has ~23,000 employees.
Pratt&Whitney, the smallest of the big three in the West, has ~40,000.
Rolls-Royce, which mostly makes aerospace engines (the car brand was sold off long ago) has ~50,000 employees.
Also ~50,000 for GE Aviation, with another 200,000 in General Electric overall.
Not all of these are engineers and scientists, but more than half the staff in such high-tech industries is in research, design, engineering, management, and other jobs that contribute to design cost.
The actual design team for a modern jet engine will be under 1,000 people. But that's just the people doing the high-level work, the flow diagrams, the FEA calculations, the design models.
 They'll rely on thousands to supply them with the data. From their models, thousands more will produce detailed drawings and CNC programs for each individual part. Then, for each individual part, a separate QC program has to be developed.
You can't simply copy-paste from design drawings to CNC programs. Nor can you copy-paste from those to measurement machine programs for QC. The measurement bases are different, so the tolerances are different, it's a different level of detail. Make that mistake just once. for a tiny and not especially critical part, and the consequences can be noticeable.

The answers are all very good in that they detail potential costs, but let me give a different angle for looking at these types of questions. In a highly competitive environment, companies will throw as much money at a problem as it is worth for them to have it solved. In economic speak: "marginal cost equals marginal gain".

When designing a new engine, one starts with all the changes that give a large performance improvement at a low cost. Over time, those "trivial" changes are explored, and if there is enough "gain" from continuous attack at the problem, more complicated changes with smaller expected gains will be attacked.
Now, think about how high the gain from an improvement of an engine is: Over the many thousand hours, how much fuel is being saved? What is the expected future market value of that fuel in a world with increasing scarcity and expected co2 taxation?
Now, consider that you will implement this new engine not in a single plane, but a  large fleet of hundreds, perhaps thousands of planes? Any improvement to the engine that you make has just such a large market value. Finally, consider that many improvements to the next generation of engines can be taken over to newer developments later on, something that is referred to as "standing on the shoulder of  giants".

An example Take an engine that costs around 30 mio USD. A performance improvement that increases the value of each engine by only 1% will be worth 300 mio USD if that engine is sold 1000 times. If that performance improvement can be reused in the next 10 generations of engines, it is worth 3 billion USD. This simple example shows you that the marginal value of R&D can get really high very quickly, and that the companies are therefore willing to throw a lot of money at these problems.

Adding to other excellent answers, I'd like to focus on the nature of the research.
The work involved in developing and exploring ideas that aren't just capable of resolving via computer models, are huge.
As other answers note, jet engines develop at the leading edge of theory and new ideas,as well as pushing existing ones.
Concrete typical example #1
Suppose we believe that a fan blade can be made stronger if it's cast in a way that it grows without certain crystalline flaws, or with a certain crystalline structure that in theory should be possible. Call it "delta form titanium-carbon crystalline matrix", or "delta-TCCM" for short.  This would allow 1.7% thinner and lighter blades,  without loss of strength or safety, or blades that can run 1.5% faster without increase in stress. If correct, this could be a big deal as part of the next generation of the current engine.
The problem is, that's as far as a model takes you. Now you need to actually achieve it reliably as a material science problem. You need to

Design a process to develop delta-TCCM reliably in a lab, which may be a huge challenge. You may need to explore multiple techniques, consider how they scale, their susceptibility to flaws and risks. The conditions for reliable low-flaw-rate delta-TCCM production may be very precise and hard to maintain for the time it takes. This can be a huge problem, far from trivial. If you don't want it to take years, you might need to throw 600 people just at the delta-TCCM research, to turn it from a concept to a usable material with verified properties.
The properties may only be somewhat predictable by theory. You may need to add trace amounts or tiny process changes, on gut feel, to resolve the issues. Each of these is a mini project in itself.
The material may be difficult to shape once formed, so you may need to go back to your lab to not only devise equipment to reliably create it, but to reliably create it to shape. Perfect shape.
You need to scale up from laboratory to industry scale. Meaning, to create enough of it to confirm properties, and to ultimately build blades. That's also far from trivial. Industry is littered with things that are easy to create in tiny quantities for research, but incredibly hard to produce at scale to the same conditions. Going reliably from  2mm2 samples without detectable crystalline/atomic-structure flaws, to curved 1.5m fan blades without detectable crystalline/atomic-structure flaws, is exactly as difficult as it sounds in many cases.
You need to test and assess ten thousand samples in a thousand ways - in isolation and in a thousand scenarios in an engine. This is a very intense process. What is its atomic structure, how does it fail (what are its modes of failure and safe limits), how does its atomic structure respond to a hundred thousand combinations/kinds/patterns of stressors, both short and long term - gaining enough understanding of the actual properties to be able to rely on them for jet safety. Perhaps go back to basics if something isn't as needed.  After all, if just one engine fails and the flaw is traced to a fundamental issue with the material, your entire reputation and product range is at risk, and refunds will be due for any sold so far, plus litigation. Your entire $500 bn business could be put at risk, in that sense, in the worst case.
You could well also build 2 or 3 entire prototype production plants (factories) in different locations, just for delta-TCCM, to confirm that you can in fact reproduce your delta-TCCM quality control reliably over time and in different establishments/sources.
Fan blades are usually made of a combination of different materials. For example the GE-9X, currently the largest turbofan engine made, uses a carbon fibre composite with steel leading edges and glass-fibre trailing edges for bird strike protection. Just making and shaping delta-TCCM isn't enough, it also needs techniques that reliably allow it to be part of a composite blade, tightly enough to retain its unified structure under all the stresses, and heating/cooling cycles, and vibrations, that are part of jet blade life. If the components don't move, shrink, and expand together, the blade may eventually weaken.
If it works, you may need to build an entire toolchain just for delta-TCCM. Machining tools, production tools, blade casts (maybe they're destructively cast and you need a new mold for every part), specialist laser or other welding, development of delta-TCCM coatings and adhesives capable of enduring in an engine environment, which are all their own independent projects. The works.

And that's just the project to commercialise delta-TCCM. $20-50 million easily, off the top (total guesswork on my part, but gives an idea). You might have 50 or 200 such projects in progress and others coming up in your R&D flow, all related to concepts you're going to explore for your new generation of engines - and all of their costs needing to be recovered by sales of the engine when finally complete.
Concrete example 2:
The space shuttle had to resist intense heat on re-entry. Far more heat than any material could possibly withstand. The idea reached was simple: ablation. The coating would burn away rather than melt, gradually exposing layers below, but not degrading as a whole.
Creating the material was a major effort. There wasn't much of a theory of such things, just a goal to create such a material. Huge research. And every time, "Good. Now find a way to do the same but making it 20% less weight".

Add to that the cost of all the ideas that sounded great until they hit a setback in one of those steps that just couldn't be overcome. All that time, money & effort for a shelved project and time to start over on a new one.

In addition to the other answers:
Jet engines are not only complex, they operate on the edge of what's physically possible. For example, modern jet engines run at internal temperatures that can be higher than the melting point of the metals used.
When you design a new jet engine, in order for it to be successful in the market, it has to be better than the engines currently available: it has to have more thrust, lower noise levels, lower fuel consumption, higher reliability, lower running costs or a combination thereof.
This means that every design moves "the edge of what's physically possible", i.e. it advances the state of the art. It's not just a new engine design, you have to develop new materials, new construction methods etc. Then you have to prove that these new developments of yours are safe to use. This is where the cost goes: scientific research (which always carries the risk that your new idea won't work as well as you'd hoped), development of the new technology to a consumer-ready level, and certification.

The problem is not fundamentally about jet engines, but about building complex things in  general.
The reasons are the same as for building a complex software. There are only gradual differences.
The question can be seen as "Why does it cost surprisingly much to create complex systems of high quality?"
The main issue is complexity. The design of existing jet engines is complex, and we know that designing an alternative is a process complex more complex than that. The same again for actually building one in series.
We want to create a complex artifact, let's name it "new jet engine".
To do this,
we need a design for it.
As base of that, we need a design specification.
To verify it, we need to build at least one instance.
In practice, we want to be actually able to create multiple exemplars for limited cost per instance.
That means we need to create multiple other artifacts too:
We need to create one or more prototypes without limiting the cost.
We need to create a complete set of tools to produce multiple instances of the artifact.
We also need to build tools to test the artifact.
We need to test one or more prototypes, and multiple produced instances based on the design specification.
We need to make an external organisation test the design specification based on certification rules.
We need to make an external organisation test instances based on certification rules.
We need to create documentation, including reliable instructions for multiple variants of service.
Note that all this is independent of the complexity of the thing we want to create. It not even depends on whether we build a physical artifact, it applies just the same for building a simulation of it, producing instances by integrating it in airplane simulation of the customer.
The many of the steps are somewhat complex in itself. When steps interact, the complexity tends to multiply instead of adding up. For example, a minor error in the design specification causes minor changes in most of the steps, and each of them has a significant overhead. Changing one screw size and the strength of one weld requires practically the same effort as only changing the screw size, because the overheads are dominating.
If we are building something complex, there are some counter intuitive aspects in terms of complexity. An important one is that the complexity and effort of testing increases very quickly for increasing quality requirements. That is in part because there are many more smaller errors than larger. It means that many more single errors need to be handled, requiring more prototypes. The overhead for handling a small error is about the same as for a large error.
To illustrate the effect of increasing quality requirements, think about building an airplane based on a plan specifying the shape and size of its parts. Compare that to an additional requirement of the total length with tolerance of a few centimeters. Now, you need to take the variation of component connections into accounts, like the distance of screws to edges of parts, and also the thermal expansion of parts. Now, refine the requirements to specify the length with a tolerance of some millimeters according to a temperature curve. Now, some tests need to be done multiple times, after finding out how many times are enough. And the differences in thermal expansion of different materials and parts from different suppliers becomes relevant. You get the point. And just in case it seems irrelevant to care about thermal expansion: The Lockheed SR-71 Blackbird actually leaked fuel when cold on ground, but did not when flying at at Mach 3.2 and about 300 °C hull temperature, based on accepted limits of precision. The Concorde became 17 cm longer in flight at about 100 °C. They had great fun when placing hydraulic lines.
Basically, adding individual parts is much more complex than the intuition would expect. Adding a part to a jet engine does not only involve stability of the part, but stability and change in shape over heat cycling, and determination of acceptable number of cycles before service.
Note that all this, apart from illustrating examples, has nothing to do with jet engines, not even whether we want to build something physical.
The specific design elements for a jet engine can be found in other answers, and a complexity estimate of them can be used to derive the over all effort here.

I think most of the answers address the points quite nicely, the teams are huge and there is a lot of expensive kit involved. I'd add three more points:

There is risk involved that needs to be priced in. It's not like in Pharma, but not all engines sell equally well, so you need to manage the costs across different engines and designs.
These are highly specialised machines, so together with a new engine your developing new tooling, new measurement techniques and new software. (There are lot's of spin offs and resultant benefits from these programs e.g.: touch trigger probe)
Just to illustrate the point of materials and manufacturing costs, these engines would be cheaper if made out of solid gold.

I happen to know the guy that designs the profile for the fan for one of the large manufacturers. He is just the academic that contributes to that design, and that is the only problem he works on. But that involved developing new software to compute the flow.

The existing answers do a great job at answering why jet engines are expensive to develop: because they are so complicated.  Let me try to answer, why are jet engines so complicated? To understand that, we have to examine the economics of jet engines, and it comes down to fuel efficiency.
Let's say that you go shopping for a new car, and one car gets 1% better gas mileage that the other.  Maybe 30 mpg and 30.3 mpg.  You'd say those are so close that it barely matters. Might as well be identical and you start to look at which one has the better sound system or the most stylish seats.  But when the airlines go shopping for new planes, 1% difference fuel efficiency is HUGE.
Airlines are large, and jet fuel is expensive.  A modest sized airline (say Jet Blue size) will spend \$1 - 2 billion per year on jet fuel alone.  And, when you buy a new plane, it will generally last for 30 years.  So over the lifetime of the fleet, the airline is spending something like \$45 billion in fuel.  If one jet engine is 1% worse in fuel efficiency, that's going to cost the airline ~ $450 million over 30 years.  That's for one percent difference in fuel efficiency.
Now there's other things that might make up for that, like the purchase price of the engine, the cost of service and spare parts, etc.  So a jet engine that's 1% worse in fuel efficiency might still be competitive overall if it makes up for it in other areas.  But beyond a few percent, the difference is so great that you couldn't even give them away.
So what you end up with is this intense "arms race" competition between the major jet engine OEMS. One company makes their engine a little bit more complicated such that they can improve fuel efficiency by a tiny fraction, and then all of the others race to catch up. This continues year after year, engine model after engine model, and before you know it what started as a fairly simple machine has been extremely complicated, and thus expensive to make.

A modest sized airline (say Jet Blue size) will spend $1 - 2 billion per year on jet fuel alone. ...Billion? Citation needed. A little math refutes this. A budget airplane, maybe 4 flights per day, 150 ppl per flight, 60 bucks per ticket. The gross revenue of that is 13.14 million per year. No way it could ever approach 1 billion. Did you mean 1 - 2 million?

@DrZ214 no I meant billion.  See for example: https://www.businessinsider.com/jetblue-raising-ticket-prices-as-fuel-costs-soar-2018-10  key quote "...fuel and related taxes rose to $515 million for the quarter".  So $500m / quarter is 2 billion per year.  Also, Jet Blue is definitely more than 4 flights per day, it's more like 1000.  Maybe you are confusing them with a different airline?

Oops, I read "airliner" and saw it as airplane, as in 1 single plane. Yeah the entire airliner could have thousands of flights per day. I'll leave these comments here in case others misread it.

@DrZ214 I quoted it that way because airlines don't just buy one engine at a time.  When they make a buying decision, they are usually committing to 10s if not 100s of engines at a time.  So the fuel costs involved in a single buying decision are large.

This is a great question! You hear this kind of question a lot on different subjects. the closer you look, the more you see the answer.
There are some great comments here already, so I'll just add a couple of points.
If you read through the design of the GE9X (here), there are pointers to the huge design cost.
There's no point in doing a redesign unless it's better. In this case, 10% more fuel efficient. This requires new knowledge, new materials, new design...
It took 8 years for deign/engineering from start to certification. They will need to run that team some years after it goers full-production to cope with any issues, which are bound to arise.
So, your core design team (could be 100 people) is looking at 10 years work. Capital equipment and support will add a lot in that time.
This will quickly swallow up $150m in-house. That's pay, not cost. Probably add 50% to house and feed, medical etc them. Each of those engineers will spend money on their part of the project. Some of them will spend a lot of money. Add on external contractors, and that could quickly looks like $$1billion.
They're using new materials which all require learning and certification. Certification is very expensive. And that's for each part!
Read the article and you'll see a single stator vane failure took 3 months to redesign, required 4 more test engines. Even at production cost, those are 45 million USD each. You can't know these things in advance. They will be built in to contingency budgets.
GE had to modify a 747 to test the engine as it didn't have the ground clearance. The had to produce 8 test engines to get full certification. That must be $800 million.
When you start adding it all up, that 10 years work is quite easily explained and divide the design cost across 10 years and it doesn't look so huge.

This is only part of the answer, but I didn't want to post it in a comment after the moderators said to keep answers out of the comments:
Look at the accounting concept of a "burdened rate." It helps to remember that you don't just need 100 engineers collaborating. You need 100 engineers in a building that has to keep the lights on and heat and/or air conditioning, with janitors and admin assistants, and all the other wonderful people who keep the engineers productive. When you factor in all these other costs of business, the hourly rate the company has to pay (as opposed to what the engineer receives) is quite different.


text:
What is engine-out taxi out time?
What is engine-out taxi out time?
a) The time between the first movement of the aircraft under its own power and the start of the last engine
b) The time between the first movement of the aircraft regardless if it's being pushed back without any engines running and the start of the last engine
And one additional question - how would you define the the start of the last engine?
a) The moment the fuel valve is opened
b) The first moment the engine is running (stable N1, fuel flow, oil pressure and temperature, etc.)
c) Something else

Where did you encounter this term? Without context any answer is a guess.

It looks like a poorly designed exam question.

It's the time taxiing without engines on, for example when under tow or during push back. As opposed to when you're taxiing under the aircraft's own power
The second is potentially subjective, but I'd personally count it as the moment the engine is actually running, post-start up procedure: that's the moment when I could use the engine as an actual engine with useful power.

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text:
What would happen if an airliner had too much fuel for its next flight?
There's an old saying that there's never too much fuel left in your tanks or too much altitude below you.
However, in terms of airline fuel planning, could there be a situation where an airliner would land with more fuel than is needed for its next flight, to the point that this extra fuel would be problematic?
Would this, or other circumstances, necessitate that fuel be removed from an airliner before a flight, for example due to a higher payload on the upcoming flight?

I have de-fueled airplanes for just this purpose, because the needed cargo or passenger payload with the current fuel weight would have exceeded the max gross weight. However, those instances were not airline flights, as your question specifies.

Those old sayings are not much use in commercial aviation.  There is such a thing as too much fuel.  Take-offs are often made with "runway behind" or with tailwind components and so on.

@RalphJ -- it can also occur if the flight plan depends on the availability of a long runway to take off, and that long runway suddenly becomes unavailable (say due to winds, debris on the runway, etal)

It can also happen when a plane scheduled (and fueled) for a long-haul flight is swapped into a short-haul flight; the plane may be under its max takeoff weight, but will be too heavy to land at its destination.  (While waiting for a flight, I overheard a captain at the next gate trying to explain a defueling delay to annoyed passengers who could not understand how a plane can be too heavy to land.)

This would be an uncommon situation and the decision on what to do will typically be handled by your dispatcher.

Take the extra gas and restrict passenger/cargo weight as necessary.
This is the easy option but it could mean leaving people behind.
Defuel to the normal amount of fuel needed (e.g. release + taxi + contingency fuel)
This normally means the fuel is considered contaminated and will be disposed of.  There are ways of de-fuelling and re-using that fuel for another plane, but it requires specific operations and equipment that only likely to be available in the hub or perhaps a maintenance station.  De-fuelling can be slow and will delay the flight.

In the end this comes down to either weight restricting the plane or taking a delay and wasting some fuel.  The timing and economics of the decision will depend on many factors and each case would be judged individually.

The people refueling the plane just subtract the amount of fuel in the plane from what the plane needs for the next trip from what I understand your question is. The other thing is that because the saying is true, airplanes come in with just enough fuel and not really enough to go anywhere else.

The question is, what happens if the result of that subtraction is a negative number.

So they would be in the side business of importing avgas?

@DJohnM Most airliners are turbine aircraft burning Jet-A (similar to kerosene), rather than AvGas (gasoline).

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text:
Does the FAA have specific Air Ambulance safety guidelines?
Does the FAA have some specific air ambulance safety guidelines to adhere to?

Not sure the question is clear: safety related to what? Piloting, taking care of the passenger?

It is unclear, but I would generally assume OP is asking about the inherent increased risk in doing things air ambulances do that normal helicopter pilots avoid, like flying in bad weather, landing practically anywhere they can which in urban environments puts them in close proximity to a number of hazards like street canyon winds, the buildings themselves, suspended wires etc, flying the most direct route possible which can intersect controlled or restricted airspace, and the technical "emergency" induced by having someone with an acute life-threatening medical condition aboard.

Check out this document published by the FAA which seems to be recent and some notes here on the same topics. The full rules can be found in the FAR at Part 135 Subpart L—Helicopter Air Ambulance Equipment, Operations, and Training Requirements
They are required to do a preflight risk analysis as per §135.617 but this is not really any different than what anyone should be doing before any flight however for them it is very specifically outlined.

(a) Each certificate holder conducting helicopter air ambulance operations must establish, and document in its operations manual, an
  FAA-approved preflight risk analysis that includes at least the
  following—
(1) Flight considerations, to include obstacles and terrain along the
  planned route of flight, landing zone conditions, and fuel
  requirements;
(2) Human factors, such as crew fatigue, life events, and other
  stressors;
(3) Weather, including departure, en route, destination, and
  forecasted;
(4) A procedure for determining whether another helicopter air
  ambulance operator has refused or rejected a flight request; and
(5) Strategies and procedures for mitigating identified risks,
  including procedures for obtaining and documenting approval of the
  certificate holder's management personnel to release a flight when a
  risk exceeds a level predetermined by the certificate holder.
(b) Each certificate holder must develop a preflight risk analysis
  worksheet to include, at a minimum, the items in paragraph (a) of this
  section.
(c) Prior to the first leg of each helicopter air ambulance operation,
  the pilot in command must conduct a preflight risk analysis and
  complete the preflight risk analysis worksheet in accordance with the
  certificate holder's FAA-approved procedures. The pilot in command
  must sign the preflight risk analysis worksheet and specify the date
  and time it was completed.
(d) The certificate holder must retain the original or a copy of each
  completed preflight risk analysis worksheet at a location specified in
  its operations manual for at least 90 days from the date of the
  operation.


text:
Do test pilots on commercial aircraft wear a parachute?
I was watching a video of the first A350 flight. At 0:35 one of the pilots appears to be wearing a helmet and what looks like a parachute. Is that true? Do pilots on maiden flights wear such a kit, so that in case of an emergency they can evacuate the airplane?

The A350 test pilots were wearing helmets, parachutes and also lifejackets.  This blog post has details and pictures of the emergency evacuation system on the A350 test flight.

@ColinPickard Note that that says "on some test flights". They don't on all. I remember seeing an A380 test (I forget what it's called), flying at V_md, where they mentioned that was the only test flight they flew with all evac gear on (otherwise it is merely onboard the aircraft in case it is needed).

To the best of my knowledge, all commercial aircraft being tested have their pilots take on parachutes and hi-vis clothing in addition to creating a possible means of egress on all important flights, such as the first flight and high-speed tests.
Since getting the cabin doors open is practically difficult, the solution I think most producers use is to built a chute down the front cargo entrance which can open.
Bombardier produced a pretty extensive film for their new CSeries.
Here's the solution employed on the Concorde:

CONCORDE! :D Parachutes sound a bit absurd on it though because you'd probably get screwed at FL 600 and at Mach 2 if something happened up there.

And, where do you fall out when you open the hatch? Wouldn't your escape be obstructed by the cargo hold?

@shortstheory it's been years since I've seen any of the prototype Concordes; my memory is under that door is a chute all the way down to the bottom of the fuselage.

Hey guys, don't worry its only an "intermediate" cover, the real cover is probably somewhere else.

Since getting the cabin doors open is practically difficult

No, that's not the (main) reason.
There's something called a wing ... which has engines mounted to it !
Not a very good idea to jump out of a door if you're likely to be sucked into an engine on your way out !

jump out using the rear exit then?

What about rear mounted engines (DC-9, most if not all private jets, etc.) Also you would be blasted by jet exhaust.

@shortstheory A forward chute could be easier to reach than a tail door, from the cockpit of a doomed pilotless plane.

This doesn't answer the question

@ptgflyer: The DC-9 has an exit in the tailcone, behind even its engines (I have no idea whether it could have been used for bailing out of the aircraft, though).

@Sean db cooper did it out of a 727 https://en.wikipedia.org/wiki/D._B._Cooper

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text:
Is the Beechcraft Bonanza Jaguar edition different from any other A36?
I've seen a few Beechcraft A36 Bonanzas with special Jaguar marketing. Some have special paint or interiors. During some hangar talk I've heard it said that the Jaguar edition is faster. Does it actually have any differences that impact performance?

From what I have read and discussed on the topic, the Jag version seems simply to be a paint job and upscale interior (not sure on the interior part). Unless Beech snuck in a 310hp 550 or 350hp TSIO the Jag should be just a dolled up A36. As far as I know, Beech only built the non-turbo A36with a 285hp 520 and then a 300hp 550, including the Jag model.

@acpilot is right- the Jaguar version of Beechcraft Bonanza differs only in the paint job and interiors. AOPA had run an article when the Jaguar versions were introduced:

Like the Jaguar edition of the Beech King Air C90B offered last year, the Bonanza and Baron will have specially outfitted interiors and unique paint schemes.
... Seats in the Jaguar airplane will use the same frames but have new cushions tailored to look like those found in the Jaguar Vanden Plas luxury car. ... tastefully wrapped the yokes in leather.
On the outside, a special paint scheme has been designed to set the Jaguar airplanes apart from standard Bonanzas and Barons.

and also,

The new soundproofing Beech plans for the Jaguar airplanes appears to work well in the rear of the airplane.

Looks like the only differences were the paint and interiors designed by Jaguar specialists:

Jaguar Cars' automotive stylists designed the paint and interior for these packages. Inside, they applied more and finer hardwoods — including a polished Jag-logo inlay on the right yoke — better carpeting, and the finest leathers from Jaguar's own vendors. Outside, the Jag-edition Bonanzas ... get a distinctive four-color metal-flake paint scheme finished off with Jaguar Cars' stylized leaping-cat insignia leaping from the vertical stab and rudder.

These are the only differences and everything else is the same. The performance changes if any would come from these- which I doubt. Around the same time, the Continental engines were refined (they were the same 300 hp though), which might have made the aircraft appear different, though there are none.


text:
Can the RAM AIR valve cause depressurization on the CRJ-900?
On the CRJ-900 AIR-CONDITIONING panel there is a RAM AIR switch light which opens a valve and allows air from the ram air scoop to enter the mixing manifold for distribution to both the flight deck and passenger cabin.
What would happen if the valve were opened at high altitude? Would this cause depressurization?
When I received CL-65 type rating I was taught that there was a protection that would prevent the valve from opening. I have also heard this was a mistake in the manuals and that it would open and cause depressurization. I have also been told that there is a one-way check valve inline with the ram air duct to prevent depressurization, but I’ve never seen a source for any of this information.
If anyone has an answer I’d appreciate citing some schematics or Bombardier manuals to back it up.
Update
I heard something. It's second-hand information from someone who worked at Bombardier when the CRJ was certified. This person said that there is no check valve on the RAM AIR valve, but the cabin outflow valves will close, compensating for the loss of pressure from the RAM AIR valve being open.
The cabin outflow valves should close completely as they are designed to prevent the cabin altitude from exceeding approximately 14,500 feet. This wouldn't prevent higher cabin altitudes if the ram air valve stayed open at a high altitude.
I still want a cited answer, but this is the first answer I've heard that makes sense to me.

If the check valves close to prevent cabin alt exceeded 14.5k ft, why would the ram air valve staying open make any difference after that. If the check valves are closed, they are closed.

The ram air port is a hole in the pressure vessel. If it's used when it is intended to be used, the cabin is at ambient pressure and it will provide some air flow. If it is used while the cabin is still pressurized, high pressure air will exit the ram air port.

No RAM AIR VALVE can open if the cabin pressure exceeds the outer pressure. For the CRJ the RAM AIR VALVE is directed so that a dynamic component of the aircraft speed helps its opening, but this is much lower than the difference of pressure between cabin pressure and the outside pressure at high altitude. So no danger of depressurization whatsoever.

did you mean it can't open if the cabin pressure exceeds outer pressure? also, what's your info source?


text:
What are the most efficient layouts of multiple runways?
Many airports have more than one runway. How should runways be built in order to achieve maximum usage and minimum delays?

Many major airports have most, if not all, parallel runways. Examples include LAX, DFW, and ATL.
In fact, Chicago O'Hare has been undergoing some major runway improvements, mainly to make the runways more parallel. This figure shows the before (mid-2000s) and after (completion date TBD) layout of ORD. Eventually, ORD is slated to have 6 parallel E-W runways, along with 2 SW-NE runways.

However, a crossing configuration is very common at airports with only 2 runways. Here is one example:

Are parallel runways more efficient, and smaller airports simply don't have space for them? Ideally, how should runways be configured?

Edit:
Additionally, what if we assume...
Case 1 - That wind isn't a factor?
Case 2 - That the airport location has exceptionally variable winds?
Case 3 - That space/airport footprint is an issue?

Surely the "efficiency" depends on what you're trying to optimize for: max throughput in the best wind, or being able to operate in many different wind directions without too much crosswind.

Chicago seems to have the best of both

Max throughput all the times.

You should also mention configurations like of SFO with sets of parallel runways crossing each other

That's a lot of structures to be demolished at ORD, and a long ride in from 28L and 27R.

@digitgopher:  I think one reason you are not getting the responses you crave is that asking for a "most efficient" layout is too confusing.  What do you mean by efficient?  Then we can provide answers.

I provided one definition of 'efficient' in the opening sentence - "How should runways be built in order to achieve maximum usage and minimum delays?" That was my question. Alternative definitions would be appreciated if that one doesn't make sense to you.

@GianniAlessandro Perhaps there is an airport location that rarely ever has much wind at all.

@digitgopher I don't see your "efficient definition". It depends on wind, on available space, on number of jet, of kind of jets: if you don't want to relate to these factors, then you already received an answer that is perfect. I don't get what you want more.

@GianniAlessandro Precisely so. This is part of where the edit to the question is going - I'd like the nuances and factors spelled out. Ideally with some sort of quantification.

In order to increase the number of operations that can be handled at a runway, you want them configured so as to allow "simultaneous independent approaches," which means that the 2 (or more) runways can be operational without the need to coordinate between them.
The biggest factor is the distance between the two runways. The greater the distance between them, the less impact they have on each other, not just in terms of number of operations per hour, but also in terms of visibility minima for instrument approaches.
The other factor that needs to be considered is the wind, and as you can see from the Chicago airport layout, there are parallel groups of runways that effectively form a triangle, such that there is never a crosswind component that exceeds 30 degrees. The layout at Las Cruces, NM, which is usually very windy, shows the format better than Chicago, even though they are single runways:

The last issue is the type of radar available, and the faster the radar sweep, the less delay there is between the position of the aircraft and their depiction on the radar screen, allowing the controllers to closely monitor the approaches.

Boston Logan is another excellent example of a triangle format.

But according to the OP Chicago has decommissioned one "side" of the triangle completely in favor of more East-West runways?

@rdp:  I am based out of Las Cruces NM.  Are you based out of there as well?

@DLH no, just dropped in a few years ago

Ideally runways do not intersect each other, and are all 4300 feet apart (laterally) to allow for simultaneous parallel instrument approaches. Denver is one such airport that meets all of this.

(flydenver.com)

This covers one aspect of "ideal", but totally misses out on dealing with crosswind. If you have a 50 mph crosswind, you can have as many parallel runways as you want but the throughput will still be 0.

@raptortech97: DEN satisfies this as well; there are four north-south and two east-west runways, none of which intersect.


text:
Would an area-ruled fuselage improve the MiG-15/17's performance?
The top speed range of these MiG 15-17 airplanes is in the transonic region, but, when they were first designed, the area rule was unknown.
Even if owning a jet fighter is not exactly 'Maintaining pilot's performance', thus hard to obtain or license, the change looks not too complex to implement; curiosity, experimenting, is the force behind Aviation progress. Salut +

In a word, yes. But that sort of depends on the definition of "performance". The top speed would likely be effected only in a minor fashion, but the transonic acceleration would likely improve, as would the fuel economy.
It is worth pointing out that, largely by luck, many of these aircraft got some of the benefits of the area rule without specifically designing it. The location of the cockpit behind the intake required the fuselage to become larger in that area, especially when one considers the canopy, which ends just in front of the wing. And then the tail begins near the end of the wing and the fuselage begins to narrow behind the engine as well.

The shape of the fuselage MiG 15 and MiG 17 fighters resulted from the large diameter of the engine used, the Russian version of the Nene centrifugal turbojet.
The "area rule" has more effect on the performance of aircraft in the transonic and supersonic speed range. A well known example is the F106 Delta Dart which had a higher maximum speed (Mach 2.3) than the preceding F102 Delta dagger (Mach 1.2) mainly as a result of the application of the area rule to the design of the F106 fuselage.

I'm sure the >40% more thrust had something to do with it as well...

Yes, but don't forget that the F106 had an empty weight more than 2000kg greater than the F102's 8630 kg.

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text:
How to lose an aircraft?
I've just read this article about the disappearance of QZ8501 and was surprised by the following statement:

Even with automated beacons (we can assume these weren’t used, weren’t
  on board or were intentionally disabled) and EPIRBs, losing an
  aircraft is actually not that difficult.

Now, is it?
I was under the impression that if Breitling can make a watch with a 406 MHz distress beacon, a €70 million aircraft must surely be equipped with an automated ELT that can start to transmit automatically and whose signal can be picked up by [LE/GE]OSAR.
As it seems, US-registered civil aircraft are not required to carry 406 MHz ELTs while aircraft registered in Australia must be equipped with ELTs that operate on both 406 and 121.5 MHz (see the same reference).
Can you shed some light upon what is the current state of ELT regulations, how ELTs fitted to commercial aircraft operate in case of distress (automatic/manual?), what are the relevant regulations for EU-registered aircraft, and how can one lose an aircraft when all these systems operate?
I don't buy that “losing an aircraft is actually not that difficult” if all available technology is used (even assuming little redundancy and an adequate price range).

A general remark (not sure what the article says) but an ELT dropped in a big body of water like the ocean, is pretty hard to find. The range of the signal in water is severely limited (compared to the size of an ocean), and you need to take into account that the distance from the ocean floor to the surface which might be substantial as well.

@falstro The article is focused on non-emergency radar contact, briefly mentions that GPS is a one-way connection, and ELTs are only mentioned in the quoted bit (apparently as EPIRBs). While you raise a good point relevant to the process of actual rescue, we are only interested in locating the crash site for which the last transmission above ground should suffice. 406 MHz ELTs transmit a burst of signal every 50 seconds, which, assuming GPS data are included, should greatly reduce the search area (even without GPS, it should be approximately 3 nm).

It is now confirmed that QZ8501 wreckage has been located close to the position of the last contact.

Actually, when crossing the big pond from Europe to the Caribbean I sometimes think about how incredibly difficult it would be to find a plane back when we haven't seen a ship or plane but only water for 9 hours straight...... There is a lot of surface on the earth when you think about it....

It is certainly possible to build an aircraft using modern technology that is hard to lose. After all, aircraft have GPS receivers and passengers can watch YouTube in the cabin. All you have to do is integrate those two and send the exact GPS position to a ground station as frequently as you want.
However, the global aircraft fleet is not so capable. Some aircraft have satellite internet capability; some do not. For those that do, the internet connectivity is completely isolated from the aircraft navigation systems. (After all, you don't want any possibility that an aircraft in flight could be "hacked" by someone on the ground.)
Retrofitting an existing aircraft (which could be up to 40 years old or more) to add such capability would be a very expensive job. Retrofitting an entire fleet (with perhaps many different aircraft models) is astoundingly expensive. Existing aircraft navigation and communications systems are so good that losing an aircraft "almost never" happens (yes, it's happened at least twice this year), compared to the number of successful flights. The airlines probably consider upgrading their fleet to support this kind of continuous surveillance not worth the expense, when compared to the probability that one of their aircraft actually goes missing.

Also, making it work anywhere in the world is significantly harder than just making it work.

I thought that the SARSAT system covers the whole world and according to the data I've found, 406 MHz ELTs for aircrafts cost around a thousand US dollars which is not much, really. Is there perhaps some drawback to the system that I'm not aware of? (cc @Lnafziger)

The initial cost might be small but it's 100's of millions world wide.  Now add the maintenance and service costs and the cost, environmental and fiscal, of all the extra fuel to carry them around.  Add the limited usefulness (exactly what benefit do you think they would provide?) and it doesn't make so much sense.

@HaroldCavendish Most of them DO have ELTs, but they don't work very well under water, as you know.

No, it only happened once the already last year; QZ8501 wreckage has already been located.

@JanHudec: QZ8501 was lost, and then found. That's different from never having been lost in the first place. If we ever find MH370, that doesn't change the fact that it has been lost for nearly a year so far.

@GregHewgill: In this sense almost every aircraft that crashed enroute was "lost" for some time, so it happened more than two times last year then. But they found QZ8501 where they expected, so the existing communication systems turned to be good enough for locating it.


text:
How does ATC radar/ADS-B ground stations not get confused with signals sent from multiple aircraft?
I understand that the radar sends a 1030 MHz interrogation signal to an aircraft and the aircraft's transpoder responds using 1090 MHz. When it comes to Mode S squitter, the transmissions are unsolicited (does not require interrogation). Considering that the airspace is highly populated, there would be multiple aircrafts transmitting data on the 1090 MHz to the radar. Won't these signals interfere with each other and become a huge mess at the radar side?
(Similar is the case of ADS-B, where data would be continuously broadcasted between aircraft-aircraft, aircraft-ATC and vice versa on the 1090 MHz Extended Squitter, how are the signals prevented from mixing up with each other?)

Also how is the interrogation signal transmitter from ATC to aircraft (and also back) sent? Is Frequency modulation used here? What is the bandwidth of the 1090 Mhz channel? Does it have sidebands?

The transponder antenna is more like a primary radar dish and is highly directional. It does indeed get confused if two targets are merging (regardless of altitude). Mode S is still interrogated, only ADS-B does autonomous broadcasts, and I'm not sure how that works. I suppose it's some kind of multiple access scheme just like have with cell phones.

According to http://en.wikipedia.org/wiki/Secondary_surveillance_radar the Mode S seems to have 1Mb/s rate (each bit takes 1μs).

@falstro There is no fancy multiple access scheme in 1090ES ADS-B. It just sends out the message often enough so that is will come across. Position 2x per second, velocity 2x per second.

@Jan Hudec Say there are 100 aircrafts in a 100 mile radius (ADS-B boasts greater precision which would allow aircrafts to fly closer). All these 100 aircrafts would send ADS-B OUT every second. If atleast 2 aircrafts transmit data at the exact same time intervals, then wouldn't a proper reception be impossible? Is some form of Time Division Multiplexing used in 1090 ES (or in 978 UAT?). Note that initially ADS-B was supposed to use the VDL MODE 4 which employed STDMA (Self Organized Time Division Multiple Access). Definitely something would be used here as well

A very similar scenario: how does the cell phone tower not get confused when many people are using the cells at the same time?

@kevin While they may be similar, how does that even work??   ;-)

@Lnafziger: Magic.

The class of ATC radars that you are talking about is secondary radar.
Secondary radar works by transmitting an interrogation signal on 1030 MHz to which the transponder replies on 1090MHz. There are several modes of interrogation. For civil ATC, modes A,C and S are important.
The uplink signal (interrogation) is a simple pulse encoding (Mode A/C) or a Differential Phase Shift Key modulation (Mode S / TCAS) on 1030 MHz.
The downlink signal (replies, unsolicited squiters) is a simple pulse encoding (Mode A/C) or Pulse Position Modulation (Mode S / TCAS / ADS-B)
Originally only Mode A & C were available. Mode A and Mode C are unaddressed; every transponder receiving a Mode A or Mode C interrogation will reply*. This causes two problems,  garbling and FRUIT.
Garbling is caused by the overlap of replies from multiple transponders. A transponder replies on 1090 MHz in the from of a number of pulses. For Mode A & C, there are at most 12 pulses between a set of framing pulses. When multiple aircraft reply to the same interrogation, these pulses mix up and it becomes difficult to find out which transponder sent which pulse. This leads to false altitudes or false squawk codes.
FRUIT (False Replies Unsynchronised to Interrogator Transmission) is caused by replies that are triggered by other radars. When multiple radars operate in an area, transponders can be quite busy and the radar is easily mislead by a reply to another radar. In addition to the timing (range) being wrong, the reply can be Mode A (squawk code) while the radar interrogated Mode C (altitude). There is no way to see what kind (A or C) reply was sent.
To overcome these problems of Mode A/C, Mode S was introduced. Mode S is quite different from Mode A/C and requires a more sophisticated radar and transponder.

Most Mode S interrogations are addressed; the interrogation contains the unique 24 bit address of the aircraft it is interrogating. This reduces the probability of garbling since other aircraft will not reply
Unaddressed Mode S interrogations will solicit replies that contain the address of interrogator (radar). This prevents FRUIT since the radar can verify that the reply is correctly addressed.
Unlike Mode A/C which does not have any error checking, a Mode S reply contains a 24 bit CRC check code.
Mode S Altitude and identity (squawk) replies have their own identifier so they can be distinguished.
Mode S radars can be operated in clusters and coordinate between them  who is interrogating which aircraft at which time. This further reduces the message load and garbling / fruit probability.

Despite all the enhancements,  Mode S radars are still receiving garbled messages and FRUIT, but are much better equipped to cope with it than a Mode A/C radar due to the addressing and CRC checks.
Since TCAS works on the same frequency as secondary radar, radars will occasionally receive TCAS messages as well. But since these have another downlink format (DF 0, DF 16) they are ignored. The same holds for ADS-B (DF 17 / 18)
Garbling remains a concern. Modern Mode S radars are relatively resilient against garbling due to their narrow beamwidth and advanced degarbling processors. But other systems like TCAS, ADS-B in and Wide Area Multilateration systems with their omnidirectional antennas receive overlapping messages quite often. But when the signal strength of the messages differs the messages can be often untangled with modern digital signal processors.
In the Mode S (& TCAS) a reply can be either short (56 bit) or long (112) bit. ADS-B messages are 112 bit long. Every bits takes  1μs, and there is a preamble that takes 8μs. So a message takes either 64μs or 120μs.
Every second, a Mode S transponder sends 1 unsolicited short squitter, and at most 6.2 long squitters when ADS-B equipped.

*To reduce number of Mode A/C replies, Mode S radars will include an extra pulse when interrogating in Mode A or C, so that Mode S capable transponders will not reply to these A/C interrogations.

I agree with you on the Mode S. The 24 bit ICAO address is used to selectively target aircrafts. Lets say a radar sends out a Mode S interrogation to an aircraft. But simultaneously, a radar in an airport will be continuously receiving squitters from aircrafts upto 100 miles away right? So won't these squitters get mixed up with the Mode S reply?
Also what about the case of ADS-B 1090 ES where signals will be sent from and between all aircrafts on the same 1090 MHz every second? Won't this definitely cause a mix up? How can there ever be a proper reception then?

What about ADS-B IN and ADS-B OUT for each aircraft? Again, both are using the same 1090 MHz which only increases the chance of message collisions further.

@Clive The beamwidth of a mode S radar is quite narrow. It does not receive squitters from aircraft all the time. It will only receive squitters from those aircraft that are in the beam, and they are in the radar beam only for a brief time. The probability of overlapping messages within the beam is not that high. Secondly if the signal strength of the messages differs by about 3dB, then it is usually no problem to degarble the overlapping replies.

@Clive Initially ADS-B OUT increases the probability of message collision, it adds at most 6.2 120μs messages per second to the frequency occupation for each aircraft. ADS-B IN will not add messages, it is only listening. On the longer term ADS-B OUT/IN will reduce the need for TCAS interrogations as hybrid ACAS solutions will partially use of ADS-B for conflict detection. Currently most of the frequency occupation is caused by TCAS so the nett effect will be a reduced probability of message overlap.

With reference to ADS-B OUT/IN. I meant to ask that since both IN and OUT use the same frequency, won't they get mixed up (while the signals propagate through air)? The aircraft 1090 ES transmitter/receiver isn't like the radar right (in the sense of high directivity. It works that way for SSR on ground since they are rotating)? Or is it?

ADS-B receivers have in general antennae that cover 360 degrees. They have a much higher change of receiving overlapping messages. But as long as the signal strength difference between the overlapping messages is more than 3dB, they can usually be untangled. If not, the message will just get lost which is not a problem, there will be another one soon enough. A terminal area radar will rotate once every 5 seconds. ADS-B transmits position 10 times in that period. To have a radar like update rate from ADS-B, you can loose about 90% of the messages.

Thankyou DeltaLima for your answers. One more question. The normal transpoder responds in the direction of the received radar beam. So how does the transponder antenna do that? Won't it have to rotate to face the required direction? Also, is the ADS-B transmitter different from the transponder antenna? 1090 ES is actually an extension of MODE S transponder right? Do both use the same antenna? Can an antenna have both functions ie of broadcast antenna and of directional antenna?

@Clive The transponder does not reply directionally, the reply is in all directions. Larger aircraft have two antennae, one on top, one on the bottom. The antenna that received the strongest signal from the interrogation will be used for the reply. Usually  ADS-B transmissions are done by the Mode S transponder and go out the same antennae, alternating between top and bottom. The TCAS receiver uses a direction finding antenna for reception, but transmits omnidirectional.

"The transponder detects incoming radar signals and broadcasts an amplified, encoded radio signal in the direction of the detected radar wave." Source : http://science.howstuffworks.com/transport/flight/modern/air-traffic-control2.htm. (Probably misleading?)

@Clive Yes, definitely misleading. There are surveillance systems that do not have co-located interrogators and receivers. These multilateration systems work because the reply is in all directions.

Thanks. Also, do you know which kind of modulation is used for transmission of signals from Radar-Aircraft and vice versa? Yes, I know that the interrogation signal uses DPSK and the reply is PPM (Shouldn't it be Manchester Coding?? I think thats more appropriate.). But digital signals can't travel through space, can it? So it should use some form of analog modulation like frequency or amplitude modulation (like the LDACS use OFDM modulation. ADS-B is operating in the L band too, so is it OFDM here too)?

Let us continue this discussion in chat.


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Does the weather condition of the runway affect the braking of an aircraft with regards to wheel braking and reverse thrust?
I was on a 737 that landed on a snowy runway in northern Finland the other day. We had to wait around 30 minutes to land as they were waiting for it to stop snowing and then for them to clear the runway.
The landing was fine but it felt like they used a lot of reverse thrust when landing. Admittedly, it's been 10 years since I last flew, but it got me wondering; in the event of such  conditions, would a pilot use reverse thrust more than wheel braking?

Some information in How does landing performance differ on water vs. ice vs. paved/grass runways? --- How do aircraft land on snow? --- How much snow can cover a runway while still allowing commercial airliners to land safely?

On a 737, you have essentially two detents for reverse thrust, which for lack of a better term you could call "normal" and "maximum".  You can use levels of reverse thrust other than that by raising the reverse thrust levers to some intermediate position, but the performance numbers are all predicated on either no reverse thrust, normal detent, or maximum.
The condition of the runway absolutely affects how effective the wheel brakes are, and when there is a lot of snow coming down -- even with a freshly plowed runway, you really don't gain much by using less reverse thrust than you have available. Plus, if anything bad were to happen, it's easier to explain yourself when the flight data recorder shows that you went immediately into max reverse thrust right after touchdown & had the anti-skid cycling all the way down the runway -- you used everything you had available to stop the aircraft.
On a long, dry runway, you have lots of choices you can make. You can use reverse thrust and a long rollout to minimize brake wear; you can use no reverse thrust (if your airline permits that option; not all do) and more wheel braking to minimize noise -- which I've experienced at Frankfurt in the early morning; or you can use a good bit of brakes & reverse thrust to make an early turnoff from the runway -- nice if your parking spot is closer to the approach end of the runway than the departure end of the runway.
With contaminated runway conditions, those options boil down to, let's get this thing slowed way down as early and as certainly as we can.  Quiet isn't so much of a concern, brake wear isn't so much of a concern, taxi routing isn't so much of a concern; we'll use all the tools we have to reduce all the risks as much as we can.
So yes, you could well see more thrust reverse being used on a snowy day than you might see on a clear, dry day.  The relative contribution of the reverse thrust and the wheel brakes is one of those engineering questions that needs a lot of specific numbers to answer... just how slick is the runway?  (Braking will be less effective, so the contribution of the TR's will be higher.)  How cold is it? (Reverse thrust is more effective in cold air -- more thrust.)  How quickly did the braking get into the anti-skid cycling? (You somewhat expect the anti-skid to be cycling on a snow-covered runway -- that means you're getting all you can out of the wheel brakes, given the friction with the runway they have to work with.)  And etc.  As a pilot, the salient point isn't that X% of my stopping came from the one and y% came from the other; it's that we used them both & reached the gate, on time & with zero groundspeed!


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Can modern aircraft also "disappear"?
Regarding the events of MH370, if an modern aircraft (like the Dreamliner, or the A350) turn off deliberately all it's radio-communication under control and fly under the radar scope, is it really possible that with so many nowaday technology, there is still no way to know the whereabouts of an aircraft?
Are new fallback location beacons set?

Possible duplicate of this question

Your question does nor make sense without understanding the difference between primary and secondary RADAR. I recommend that you read about these and perhaps rephrase the question.  It is to all intents and purposes impossible for any commercial aircraft to avoid primary RADAR, if in an area of coverage, but trivially easy to disappear from secondary RADAR.

And primary radar coverage is not perfect world wide

Terrain masking is a technique used by military aircrafts to avoid (primary) radar.

Also note that the 777-200ER IS a modern aircraft... the design was less than 20 years old and the airframe was less than 12 years old... It's not the very latest aircraft, but it's pretty new.


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Would it have been feasible to install 'conformal fuel tanks' on the Mercure?
The Dassault Mercure was a short-range twin-jet airliner. It was canceled with sales not reaching the breakeven point. Other similar jetliners like the Boeing 737 had a more convenient longer range.
The structural design features of the Mercure precluded increasing the fuel capacity, but there are jet fighters where conformal fuel tanks were added, same as in the Hypersonic experimental X-15.
Are this type of added fuel tanks a feasible option for increasing the Mercure range?

It's rather confusing to put a picture of the X-15 and then have the Dassault Mercure as the first plane mentioned. I think it makes sense to put a picture of the Mercure at the top, and only put the picture of the x-15 once you introduce the concept of conformal fuel tanks.

I counted on the premise that you can read, also that you are not blind. Sorry if I disoriented you. Salut +

Adding conformal tanks will add drag and weight. Each on its own will impact performance but both together will definitely have a huge impact and I doubt that it would even get airborne.
The reason that jet fighters have these is for ferry flights eg deployment overseas.. and since they usually leave the bombs/missiles/etc behind on these ferry trips there is not a big increase impact in take-off weight.

Wikipedia article suggests no:

After the commercial failure of the Mercure 100, Marcel Dassault requested of his engineers to develop a new version of the Mercure, the Mercure 200C, in cooperation with Air France. The variant was to carry a maximum of 140 passengers across a range of 2,200 km. Several major airlines in the United States showed some interest in the project. However, the project's design costs were also high. This might have been mitigated if the original Mercure had had a larger fuel capacity or sufficient design strength so that additional fuel tanks could have been easily added.

Why the downvote?

The issue may be in normally, fuel tanks are at least in part in the wings, thus enlarging fuel load requiring an strenghtening of wing structure, that adds weight in wing, However, external fuel tanks can be placed as not changing gravity center, not adding much load on fuselage, if the added weight is feasible for the airplane to carry without major modifications was part of my question. For sure, X-15 did, and this is a very special and 'delicate' aircraft.


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When is Minimum Safe Altitude Warning triggered?
I know that Minimum safe altitude warning (MSAW) alerts ATC and pilots if an airplane is under the risk of controlled flight into terrain.
But when is this alarm triggered? What is the minimum altitude for it to trigger? If a plane is landing, should it be triggered just before landing?
It will confuse ATC if every plane that is landing triggers that alarm.
There must be some limitations. So, what are they?

I think I understand the question(s) here, but paragraph 2 has me, maybe, confused.

note that Part 91, 135, and 121 all require require onboard terrain awareness device for turbine aircraft of 6 or more pax, so ground radar-based warnings are less important today than they once were EG:  http://www.law.cornell.edu/cfr/text/14/135.154

I think you are trying to refer to a terrain warning system in the ATC facility. If so, you should clarify this in your question.

MSAW will be triggered if the altitude is already low, but also if the predicted altitude, based on current parameters, is low too. See SKYbrary

MSAW will not alert a pilot directly but rather just ATC. As mentioned above, the pilots will have a TAWS or GPWS system onboard. If you are interested in the flight conditions that will cause such a system to create an alarm to the pilots, this FAA document has the test conditions listed.

I'm sorry I don't understand the question.  Is your question from the point of view of an airline passenger with concerns?

The system projects the altitude that an aircraft will be at based on recent track and a calibrated "look ahead time" and an expected controller response time.  That value is then compared to a calibrated altitude floor.  If the aircraft is projected to be below the floor, then the controller will be alerted.
The specific values for all those things are calculated to minimize false alarms, accommodate the type of operation and local operational needs.  The values are specific to that facility and that area and so a straight answer isn't exactly possible.
Here are a few interesting resources in case they help.
Minimum Safe Altitude Warning
EUROCONTROL MSAW Guide

I have rolled this back to revision 2. The name is correctly spelled EUROCONTROL.

Thank you.  I had just made some formatting changes which it looks like you retained.  How did I spell it?

EuroControl, which was fixed in rev2 and undone by you in rev3, looks like by accident if you were doing edits.

Yep.  I just added line breaks to the links.  Didn't like the way they ran up against each other.

MSAW is one of a family of ground-based safety nets designed to provide warnings to air traffic controllers.
There is not a standard or performance specification for MSAW, so the features of each system varies depending on the system manufacturer, influenced sometimes by the needs expressed by the ANSP (Air Navigation Service Provider).
Many modern MSAW systems store the alerting surface as a grid of elevation values which represent the terrain (i.e. a terrain model). A margin above the terrain (typically a few hundred feet) is often included in the terrain model. Almost invariably, MSAW applies a simple linear prediction to determine whether each aircraft under ATC will be lower than the terrain model within a specified warning time or a 'look-ahead time'.
Some older MSAW systems which have no prediction are probably still in operational use - in these cases the MSAW surface may be set slightly higher, although the balance between warning time and nuisance alert rate may be harder to get right.
MSAW inhibition areas are defined to encompass the final approach paths to runways which allows aircraft to land at airports/airfields without triggering an MSAW alert. If APM (Approach Path Monitor) is available, then it often makes sense for the APM approach funnel and the MSAW inhibition area to have the same dimensions so that there is no gap in the protection afforded by the ground-based safety nets.

Hi Rod. This answer would be much easier to read if it was broken up into paragraphs. Can you [edit] your answer to add some blank lines here and there, wherever applicable? Doing so will break it up into paragraphs.


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What are the benefits of a pivot wing?
The answer to another question mentions the rather unusual pivot wing. From an aerodynamical standpoint, what are the benefits of an oblique wing compared to a delta, a trapezoid or a forward swept wing?
Are there any other differences?

Note that I am asking about the oblique wing when it is oblique. The advantages of variable-sweep aircraft have already been discussed at length.

related:http://aviation.stackexchange.com/questions/19130/what-are-the-advantages-and-disadvantages-of-an-oblique-wing-over-a-swing-wing

The pivot wing, also called oblique wing is a type of wing design that tries to minimize the drag over the entire speed range of the aircraft.
At low speeds, the main concern is the induced drag, which can be reduced by increasing the aspect ratio. That is why low speed aircrafts like gliders have very long slender wings.
As the speed increases, the induced drag falls off quickly and the wave drag comes to dominate at supersonic speeds. This is the reason why supersonic aircrafts have highly swept wings with very small aspect ratios.
One way to reduce the drag at high speeds is to sweep the wing. The velocity component perpendicular to the wing is reduced, reducing the drag.
The delta wing is a highly swept wing used in supersonic combat aircrafts. A good example is the Eurofighter Typhoon, which is a tailless delta. The delta wing provides excellent supersonic performance and has a high stall angle. However, the wing 'bleeds' energy rapidly during turns and the requires high takeoff and landing angles.

"Eurofighter Typhoon line drawing" by Inductiveload - Own work. Licensed under Public Domain via Commons.
The trapezoidal wing is a  version of high speed wing with small aspect ratio. The most famous example of the use of trapezoidal wing was the Lockheed F-104 Starfighter, which had excellent high speed performance. However, the aircraft had high takeoff and landing speeds and comparatively less low speed performance.

"Lockheed F-104C Starfighter" by Kaboldy - Own work. Licensed under GFDL via Commons.
Almost all the swept wings in use are swept towards the rear. One disadvantage of this kind of wing is the th airflow is from the root to the tip and as a consequence, the (outboard) ailerons stall before the inboard portion of wing and this results in loss of roll control in high angles of attack. This can be prevented by sweeping the wing forward. However, the bending stress in the wing root is increased due to aeroelastic twisting.

source: NASA
The issue with fixed sweep wings is that what is efficient in one regime will not be so in the other. One way to reduce the drag in all speeds is to change the sweep and aspect ratio of the wing.
The variable sweep wings change the aspect ratio by rotating a part of the wing.

"Three F-111s with different wing configurations" by Jason B from Australia - 3 Configs. Licensed under CC BY 2.0 via Commons.
The pivot wing is an extreme version of the swept wing where the entire wing is rotated. So far, it has only been implemented in one aircraft, the NASA SD-1.

"AD-1 ObliqueWing 60deg 19800701" by NASA - http://www1.dfrc.nasa.gov/Gallery/Photo/AD-1/HTML/ECN-15846.html. Licensed under Public Domain via Commons.
The main aerodynamic advantage of the oblique wing is that by adjusting the sweep progressively, the aircrafts can be optimized for drag reduction over a wide range of Mach numbers. Also, the AD-1 was designed with elliptical wing for minimizing induced drag.
However, on the downside, as the entire wing is rotated, one side of the wing is swept forward, while the other side is swept towards the rear. This means that their stalling characteristics are entirely different. On one side (forward swept side), the inboard region stalls first, while its the opposite on the other side. This makes controlling the aircraft difficult. A follow on project of AD-1, the Northrop Grumman Switchblade was canceled over control difficulties.
The aircraft also exhibited aeroelastic and pitch-roll coupling effects that adversely affected the aircraft's handling qualities at over 45 degrees sweep.
Also, the aircraft exhibited unusual trim requirements and inertial coupling. For instance, it was reported tha the AD-1 required about 10° of bank in order to trim the aircraft with no sideslip at 60° wing sweep.
Though the pivot wing aircraft had excellent aerodynamic characteristics, its poor and unusual control characteristics prevented further development.

The pivot wing and the variable sweep wing both enjoy the same benefits to any non variable wing design which is the ability to adjust the sweep angle and aspect ratio of the wing and the relative thickness of the airfoil. All of which helps to optimize the wing for different airspeeds. When flying at low speeds, a high aspect ratio keeps the induced drag low, which takes up the majority of drag. In addition, the greater relative thickness gives the airplane the ability to fly at relatively low angles of attack which helps when landing or taking off and keeps drag low. At higher speeds especially when approaching Mach 1 wave drag takes up the majority of drag. To reduce wave drag thin wings with a low aspect ratio are required.
The aspect ratio is the ratio between the wingspan of the aircraft and the wing area. The wingspan changes with sweep angle on a variable sweep or pivot wing, whereas the wing area stays the same. This causes the lower aspect ratio of the swept wing.
The relative thickness is the ratio between the thickness of the airfoil and the chord line.

To get the chord line you measure from the leading to the trailing edge of the wing. When you do this in swept wing configuration you will find out that the chord line is longer than when the wing is straight. Therefore the thickness ratio is lower when the wing is swept.
Trapezoidal wings like those on the F-104 Starfighter had very bad low speed characteristics and could not hold any fuel because they were so thin. To achieve enough lift high angles of attack as well as a high landing and take off speed were needed. At high speeds however these wings had very low drag enabling the Starfighter to fly Mach 2 even though its engine was relatively weak compared to modern ones.
The delta wing achieves the low supersonic drag by choosing a higher sweep angle. It is easy to build and fuel tanks can be placed inside the wing. The delta wing has a very high stall angle, so that high lift coefficients for slow flight or for tight turns are possible. This is because vortices along the wings leading edge create additional lift at high angles of attack. However, that comes at the cost of extremely high drag. Depending on the sweep angle it can be chosen for which speed the wing is optimized.
Todays fighters use all kinds of tricks in order to adapt the wing to the desired airspeed. One example is the use of strakes. Strakes are basically wings with a higher sweep angle than the rest of the wing placed in front of the actual wing to create the beneficial vortices without having to compromise with a low aspect ratio. The advancements in those and other designs as well as more powerful engines have made the variable sweep wing requiring complex sweep mechanics obsolete.
The main advantage of a pivot wing compared to a swept wing, that I can think of, is the lower trim drag of the pivot wing when the wings are swept. While pitching moment changes with sweep angle for variable sweep wing designs, it remains the same for pivot wing designs.
On a typical variable sweep wing design the aerodynamic center moves aft when the wings are swept back. This is because most of the wing surface moves aft when the sweep angle of the wing increases. The aerodynamic center is the point on which all aerodynamic forces of the aircraft are imagined to act upon. So When the aerodynamic center of the wing moves back the distance between the center of gravity and the aerodynamic center will get larger (for stable aircraft designs).
Flying supersonic moves the aerodynamic center even more towards the tail.
The torque around the pitch axis (pitching moment) acting on the aircraft can be calculated by multiplying the total force on the vertical axis (basically lift) by the distance between the aerodynamic center and the center of gravity.
To compensate for the greater magnitude of pitching moment more positive trim needs to be applied when the wings are swept back in order to maintain level flight or even to pitch up. More trim results in more drag.
The difference in pitching moment is why many early variable sweep wing designs had longitudinal stability problems.
To compensate the difference in pitching moment the F-14, for example, introduced little retractable lifting surfaces in front of the wings in its later versions. These would extend when the wings were swept to move the aerodynamic center forward reducing the trim drag.

On a pivot wing design the aerodynamic center remains exactly the same no matter the wings sweep angle.
When pivoted, the aerodynamic center of one wing will move a certain distance to the rear, and the aerodynamic center of the other wing will move an equal distance to the front. Thus the aerodynamic center of the entire wing will not move when the wing is pivoted and the pitching moment remains the same.
No additional trim is required on a pivot wing for changing the sweep angle and thus no additional trim drag. However, a pivot wing has numerous other major aerodynamic problems as have been depicted in other answers and is therefore not really practical for aircraft in operational service.

I think the main difference between a swept and a straight wing is that the straight wing has a lower landing speed, whereas the swept wing can fly closer to the speed of sound without compressibility problems. So a pivot wing gives the benefit of both, the disadvantage being extra weight of the pivot mechanism.
Peter Garrison explains the benefits of a swept wing here: http://www.flyingmag.com/why-are-wings-swept

So when the pivot wing is at a 60 degree angle for instance, it behaves the same way as a delta wing with the same angle?

Assuming the pivot wing is exactly the same shape as the delta when its swept back I don't see the difference.

It's not exactly the same, as forward sweep and aft sweep are not the same, both structurally and aerodynamically, see this question:
http://aviation.stackexchange.com/q/2360/4197

I don't think I'm qualified to answer swept vs delta. For sure these are different. The delta has higher drag at higher angles of attack so often doesn't have flaps because the pilot can just lift the nose to slow down.

@PhilipJohnson but they aren't the same shape. The pivot wing looks like a straight wing, only turned at an odd angle. The delta wing looks, well, like a capital greek letter delta.

@ROIMaison when the pivot-wing is pivoted, the wing on one side is forward swept and the wing on the other side is aft swept. Does this mean that the stall characteristics are very different on each side of the plane?

Yes, exactly, if the wings are exactly the same shape, stall will happen first on the inboard side of the forward swept side, and outboard on the aft swept side of the aft swept side, which will lead to a very asymmetric aircraft.

For the interested people, I posted a related question comparing swing wings with oblique wings : http://aviation.stackexchange.com/questions/19130/what-is-the-advantages-and-disadvantages-of-an-oblique-wing-over-a-swing-wing